1. Why are siblings so different
Why are siblings so different? Exploring genetic versus family influences on personality.
Intro Personality
Feb
Two things:
Siblings tend to look alike.
They tend to be very different in personality.
This lecture will only be covered by the final exam. It will not be on the conflict/midterm Thurs March 1st

2. Questions:
What role do genetics play in the development of personality traits?
What aspects of the environment are important in the development of personality traits?
Genes vs. Environment

3. Heritability of height = 90%;
Age 18 pictures
Height: .9
Weight: .9
Heritability of height = 90%;
Weight?

4. Richard vs Roland Early Temperament: Easy vs Difficult
Neuroticism Richard > Roland
Extraversion Richard < Roland
Openness Richard = Roland
Agreeableness Richard > Roland
Conscientiousness Richard < Roland
Internal Locus of Control
Personality: different from bother.
Richard:
More neurotic and agreeable
Less extraverted (in dominance) and conscientiousness
Equally open

5. Heritability Quotient:
An estimate of the proportion of variability in a given characteristic that can be attributed to genetic differences between people.
Can range from 0 to 100.
Involves population

6. Methods to explore heritability estimates for personality traits
Twin Studies
Fraternal twins (DZ) share 50%
Identical twins (MZ) share 100%
Adoption studies
Adopted sibs share 0 genes
Biological sibs share 50%
Combined Adoption/Twin Studies
If identical twins: it doesn’t apply.
Personality may be very similar.
Natural experiments;
For dz’s use same-sex

7. If genetic endowment is important, you would expect:
1. r’s of mz twins > r of dz twins
2. r’s of bio sibs > r’s of adopted sibs.
“r” is symbol for correlation;
mz = monozygotic or identical twin;
dz = dyzygotic or fraternal twin.

8. Amazing similarities of identical twins who are raised apart
Jim Twins:
1st wife “Linda”
2nd wife “Betty”
Son “James Allen”
Dog “toy”
Part-time cop
Drive Chevy
Chain-smoke same brand
Drink same beer
Vacation in FLA

9. Identical Twins Reunited
Show Film Clip

10. Major Studies Comparing Extraversion of MZ and DZ Twins
Other Big 5 Traits?
MZ DZ
Sweden US
Canada
Germany
Approximately 50% heritability
= .25
Most of us think that nurture is more important.
Mostly think things are a results of the family upbringing.
Identical = about .25 greater
Heritability quotient: 2 times the difference between MZ and DZ
HQ = 2 X (MZ r – DZ r)

11. Twins Raised Together versus Apart
DZ together, r = .19
DZ apart, r = .14
MZ together, r = .52
MZ apart, r = .49
Extraversion: r with adoptive parent = .00, r with biological parent = .21
Can’t make the second assumption, have to look carefully at twins and then make influences of shared environment.
Being raised in the same family doesn’t make people more similar.
Extraversion: 0 with adoptive parent, .21 with real parents.
Shared environment only accounts for only a small amount of variance.

12. Two surprises in these numbers?
1. MZ's are often more than 2 X as high as DZ's.
2. There is little evidence for role of shared family environment.

13. Translation into Everyday Language By McAdams:
1. Identical Twins are more similar to each other than expected.
2. Biological siblings are less similar to each other than expected.

14. Non-additive genetic variance:
Genes may not influence traits in a linear, additive way, but rather combine and interact in a “configural” pattern in which all components are essential.
The absence or change in any one gene can produce large quantitative changes in the result.
- Biological siblings for most traits have many genes involved and to have the same personality you would need the same configuration. Because it’s not additive, sharing 50% of genes doesn’t contribute much to similarity.

15. Next Question
If 50% of the variation is genetic does that mean the remainder is the result of the environment, specifically parenting?

16. Genetic and Environmental Variance
Shared environment:
influences that operate to make family members alike
Non-shared envir:
influences that operate to make family members unalike
At best 5% for shared environment: same house, same sister, etc.
Non-shared environment: 35%, influences that make you different (friends, one is the favorite, teachers).
Rest: measurement error
50% for E?

17. McAdams Quote:
“In teaching personality psychology for 25 years, my experience has been that students (and many colleagues) simply do not believe these results!”

18. Non-Shared Environmental Influences:
Perinatal Trauma
Accidental events
Family Constellation
Differential parental treatment
Differential sibling interactions
Events Outside of the Family
Pregnancy: medical problems, etc.
Accidental events
Family constellation: can be VERY important
Parents treat children differently. Very little agreement between the children. Exception: identical twins.
Differential sibling interactions – how do you use siblings?
Things happen outside of the family. Peer group. Family not that important after age 10.

19. Birth Order Effects:
Study that examined individuals within the same families
First borns higher on achievement and conscientiousness;
Younger siblings higher on rebelliousness and agreeableness;
Paulhus et al., 1999

20. Adding complexity: Extreme home environments;
More influenceable traits:
Empathic concern;
Delinquency;
Locus of control;
Not well defined (the non-shared environment).
Almost all studies assume we were raised in similar environments.
However, neglectful environments have effects.
Meanwhile, no ideal family environment.
More influencable traits:
Empathic concern
Delinquency
Locus of control

21. Questions:
What role do genetics play in the development of personality traits?
What aspects of the environment are important in the development of personality traits?
You share 50% with your siblings but that doesn’t give you much in terms of similarity.
Same parental influences but that doesn’t account for much either.